1. Field of the Invention
This invention is concerned with an improved process for converting synthesis gas, i.e. mixtures of gaseous carbon oxides with hydrogen or hydrogen donors, to hydrocarbon mixtures. More specifically this invention is concerned with an improved process wherein the fluidity of the catalyst suspension used in a slurry phase Fischer-Tropsch process is maintained.
Processes are well known for converting coal and other hydrocarbons, such as natural gas, to a gaseous mixture consisting essentially of hydrogen and carbon monoxide and/or dioxide. Those of major importance depend either on the partial combustion of the fuel with an oxygen-containing gas or on the high temperature reaction of the fuel with steam, or on a combination of these two reactions. An excellent summary of the art of gas manufacture, including synthesis gas, from solid and liquid fuels is given in Encyclopedia of Chemical Technolgy, Edited by Kirk-Othmer, Second Edition, Volume 10, pages 353-433 (1966), Interscience Publishers, New York, N.Y. and in the more recent Third Edition, Volume 11, pages 410-446 (1980), John Wiley and Sons, New York, N.Y.
It is also well known that synthesis gas will undergo conversion to reduction products of carbon monoxide, such as hydrocarbons, at from about 300.degree. F. to about 850.degree. F., under from about one to one thousand atmospheres pressure, over a fairly wide variety of catalysts. The Fischer-Tropsch process, for example, which has been most extensively studied, produces a range of liquid hydrocarbons, a portion of which have been used as low octane gasoline. Catalysts that have been studied for this and related processes include those based on iron, cobalt, nickel, ruthenium, thorium, rhodium and osmium, or their oxides.
Recently, it has been discovered that the conversion of synthesis gas into valuable products can be greatly enhanced by employing a special type of crystalline zeolite exemplified by ZSM-5 in admixture with a carbon monoxide reduction catalyst. Thus, for example, in U.S. Pat. No. 4,086,262, there is disclosed a process for the conversion of syngas by passing the syngas at elevated temperature over a catalyst which comprises an intimate mixture of a Fischer-Tropsch component and a special type of zeolite such as ZSM-5. This patent points out that the products produced are hydrocarbon mixtures which are useful in the manufacture of heating oil, high octane gasoline, aromatic compounds, and chemical intermediates.
More recently it has been discovered that a highly aromatic or highly olefinic gasoline of enhanced octane number, or a gasoline plus distillate mixture, can be obtained in greater yield from synthesis gas utilizing a selected synthesis gas composition of low H.sub.2 /CO ratio in a relatively special Fischer-Tropsch syngas conversion operation and in a sequentially arranged dual reactor conversion process. Such a process is described in U.S. Pat. No. 4,279,830, which is incorporated herein by reference. The process basically is a two-stage process which consists in the first stage of reacting the syngas mixture in the presence of a special Fischer-Tropsch CO reducing catalyst under preselected conditions. The gaseous product obtained from this first stage syngas conversion is thereafter in the second stage processed in a second reactor with a special crystalline zeolite catalyst of a desired activity to yield a synthetic hydrocarbon product containing a gasoline fraction rich in aromatics.
In conjunction with the Fischer-Tropsch process, there has been developed more recently the slurried catalyst reactor system. This can otherwise be described as a suspended Fischer-Tropsch catalyst in a liquid medium suitable for the purpose of converting syngas to hydrocarbon products. The slurried catalyst reactor system is discussed in U.S. Pat. No. 4,252,736 which is incorporated herein by reference. This particular reactor system is also discussed extensively in the article, "Fischer-Tropsch Synthesis in Slurry Phase", M.D. Schlesinger et al, Industrial Engineering Chemistry, Volume 43, Number 6, page 1474 (1951). Basically the slurried catalyst process constitutes a process in which a finely divided iron catalyst suspended in oil is circulated by natural convection through a reactor in the presence of synthesis gas. U.S. Pat. No. 4,252,736 discloses a process in which synthesis gas is first bubbled through a column of Fischer-Tropsch catalysts suspended in oil. The effluent is then flowed through a bed of zeolite (ZSM-5) and hydrocarbons boiling in the range of gasoline and distillate fuels are recovered from this second effluent.
In any process using a slurried catalyst to convert the syngas to higher molecular weight hydrocarbons, it has been noted that with the passage of time the slurry of catalyst becomes increasingly viscous until the slurry approaches gellation at reaction conditions. When this condition prevails, the process must be discontinued and at least a portion of the slurry must be replaced by a more fluid suspending agent. This tendency of the catalyst slurry to become thicker is thought to result from the formation and accumulation of heavier hydrocarbon waxes of C.sub.30 + composition. The formation of these heavier hydrocarbons results in loss of product as well as loss of production time. A primary object of this invention accordingly is to prevent the buildup of heavier hydrocarbons in the catalyst slurry in a Fischer-Tropsch process.